Dithiooxamide (C.sub.2 H.sub.4 S.sub.2 N.sub.2) is a well-known and widely studied material of the structural formula I, as follows: ##STR1##
Dithiooxamide is also a well-known ligand for complexation or coordination with transition metal cations. In particular, it is known to coordinate with such cations as Ni.sup.+2, Zn.sup.+2, Pd.sup.+2, Pt.sup.+2, Fe.sup.+2, Cu.sup.30 2, and Co.sup.+2.
Certain substituted dithiooxamides are also fairly well-known and widely studied. These are certain N,N'-(disubstituted)dithiooxamides according to the general formula II as follows: ##STR2## wherein R.sup.1 and R.sup.2 are a variety of alkyl or aryl groups, including straight chain or branched structures, and chains including heteroatoms therein, such as oxygen, nitrogen, or sulfur.
N,N'-(disubstituted)dithiooxamides, i.e., compounds according to formula II, are theoretically capable of forming a variety of types of complexes with transition metal cations. Three of these will be referred to herein as a "monomer" complex, a "cationic" complex, and a "polymer" complex represented by the formulas III, IV, and V, respectively. ##STR3##
The monomer complex can be generally represented by the formula M(HL).sub.2 wherein M is a transition metal cation having an oxidation state of +2 and HL is a dithiooxamide ligand according to the general formula II, having an overall charge of -1 due to removal of one thioamide hydrogen therefrom.
The polymer complex comprises a one to one coordination between ligand and transition metal cation and can be generally characterized by the formula (ML).sub.n wherein M is a transition metal cation having an oxidation state of +2 and L is a dithiooxamide ligand according to formula II, wherein two thioamide hydrogens (one from each nitrogen) have been removed resulting in an overall charge on the ligand of -2.
The "cationic" complex is represented by the general formula M(H.sub.2 L).sub.2 X.sub.2 wherein M is a transition metal cation having an oxidation state of +2, H.sub.2 L is a dithiooxamide ligand generally represented by formula II above, and X is an anion having a charge or valence number of -1.
Formulas III, IV, and V are for the purpose of representing stoichiometry only. The coordination of the ligands may vary, thus allowing for a variety of isomers.
The polymer complexes of N, N'-(disubstituted)dithiooxamides are known and have been studied. In part, this is due to the fact that they exhibit a strong, typically magenta, color. As a result, they have been commercially used in products such as carbonless paper constructions.
Generally, a carbonless paper construction comprises two substrates, for example two sheets of paper, each with one face, or side, coated with a reactant. The two substrates are generally referred to as a donor sheet and a receptor sheet. When the coated faces of the two substrates come into contact such that the reactants can mix, a reaction occurs and an image forms.
In a typical carbonless transfer paper construction, the donor sheet generally includes thereon a coating of encapsulated ligand material in a suitable binder, and is often referred to as the coated back (CB) sheet. The receptor sheet generally includes a coating of a transition metal salt optionally in a suitable binder, and is often referred to as the coated front (CF) sheet. The metal salt is such that it is capable of forming the polymer when brought into contact with the dithiooxamide ligand. In use, the donor sheet and receptor sheet are placed with the CB surface in contact with the CF surface. That is, the two sheets are positioned such that the capsule coated donor (CB) sheet faces the metal salt coating on the receptor (CF) sheet. Stylus pressure (for example pen pressure) or key pressure (for example typewriter key pressure) applied to a frontside of the donor sheet, i.e., the uncoated face, will tend to break the capsules containing the ligand, releasing same to transfer to the surface of the receptor sheet and form a colored complex with the metal salt thereon. With respect to this, attention is directed to G. W. Matson, U.S. Pat. No. 3,516,846 (1970). The appropriate stylus or key pressure will be generally referred to herein as "activating pressure." Such systems are generally referred to as "carbonless paper" arrangements, since a copy of what is printed or typed onto the donor sheet is generated on the receptor sheet without benefit of carbon paper.
As explained above, generally N, N'-(disubstituted)dithiooxamides form magenta polymer complexes with nickel. This is less than ideal for carbonless paper products, since darker blue or blue-black colors are preferred for both aesthetics and contrast. Therefore, alternatives to N,N'(disubstituted)dithiooxamides have been sought as ligands for use in such products or for use in any applications in which a blue image is desirable.
It has recently been discovered that certain N-(monosubstituted)dithiooxamides react with transition metal cations to form dark, i.e., blue or blue-black, colored polymer complexes. With respect to this, see copending U.S. application Ser. No. 07/438,776. It is noted that mixtures of both N-(monosubstituted)dithiooxamides and N,N'-(disubstituted)dithiooxamides may be useful for carbonless paper.
The present application concerns preferred methods for the synthesis and isolation of N-(monosubstituted)dithiooxamides. As will be apparent from the following descriptions, conventional methods for synthesizing substituted dithiooxamides have not been completely acceptable for achieving this end.
The reaction of unsubstituted thioamides with primary aliphatic amines, known as the Wallach Reaction, has been used in the preparation of substituted dithiooxamides. See O. Wallach, Ann. 1891, 262, 324. In particular, a variety of N,N'-(disubstituted)dithiooxamides have been prepared by this method; see for example R. N. Hurd et al., J. Org. Chem. 1961, 26, 3980.
The Wallach Reaction is a transamination reaction. It is limited in that aromatic primary amines, such as aniline, and secondary aliphatic amines, such as diethylamine or diethanol amine, are unreactive under the typical reaction conditions. Additionally, the Wallach Reaction generally favors formation of the symmetrically disubstituted product, and monosubstituted products are difficult to isolate in significant yields. See, for example, G. H. Alt, U.S. Pat. No. 3,658,900 (1972) and B. Persson et al., Acta Chem. Scand. 1964, 18, 1059.
A variation of the Wallach Reaction has been used to favor formation of N-(monosubstituted)dithiooxamide compounds. In particular, Haske et al., J. Org. Chem. 1967, 32, 1579, have reported preparation of several aliphatic-substituted dithiooxamides according to a one-step synthesis that makes use of the acidic nature of the thiooxamide proton. In general, Haske et al. proceeded through blocking one of the thioamide functions, by formation of the dithiooxamide salt. Reaction of the salt with an aliphatic amine was reported to give a low yield of monosubstituted dithiooxamide. Via their process, they reported the preparation of N-ethyl, N-propyl, N-butyl, N-3dimethylaminopropyl, N-2-hydroxyethyl, and N-3-hydroxypropyl derivatives of dithiooxamide. Typically, yields ranged from about 5.7% (for the N-butyl compound) to 15.3% (for the hydroxyethyl compound). The reactions, which were carried out in aqueous sodium hydroxide, were typically accompanied by hydrolytic decomposition of the dithiooxamide, leading to formation of ammonia, sulfide, cyanide, thiocyanate, and oxalate ions. Attempts to minimize hydrolysis by shorter reaction times typically resulted in increased recovery of starting material.
Alternative methods for preparing N-(monosubstituted)dithiooxamide derivatives are known. For example, see H. U. Kibbel et al., J. Prakt. Chem. 1981, 323, 41, and G. Erfurt et al., D. D. R. Patentschrift 112,435 (1975). This synthesis is based on reacting sodium cyanodithioformate with an amine, followed by treatment with hydrogen sulfide to form a dithiooxamide. The toxicity of hydrogen sulfide, the handling and filtering of cyanide solutions, as well as the flammability of carbon disulfide used in this reaction, serve to make the reaction undesirable for use in large-scale production for commercially useful monosubstituted dithiooxamides.
Other routes for the preparation of monosubstituted dithiooxamides include the use of various toxic and hazardous materials, which create the same problems as mentioned above with respect to commercial production. For example, phosphorus pentasulfide has been used in the treatment of N-phenyloxamide to prepare N-phenyldithiooxamide; see A. Reissert, Chem. Ber. 1904, 37, 3708. Also, isothiocyanates have been used in reaction with potassium cyanide, the product of which was then reacted with ammonia and hydrogen sulfide to form various N-(monophenyl)dithiooxamides. See A. Reissert, Chem. Ber. 1924, 57B, 981 and A. D. Grabenko et al., Zh. Org. Khim. 1972, 8, 528.
It is noted that the methods reported above, specifically developed to favor preparation of monosubstituted dithiooxamides, have only been used to prepare compounds substituted with relatively short-chain alkyl groups or aryl groups with short-chain substituents. That is, methods for the preparation of N-(monosubstituted)dithiooxamides, wherein the groups substituted on the nitrogen are sufficiently large to render substantial nonvolatility in the monosubstituted product, and thus potential useability in commercial applications such as carbonless paper products, have not been shown in the literature. The need for the preparation of such compounds exists due to the commercial utility of products with a blue or blue-black image.
It would be preferred that the methods of synthesis be such as to render: favorable, or at least commercially acceptable, overall yields of monosubstituted materials; favorable, or at least commercially acceptable, ratios between monosubstituted material and disubstituted material; product mixtures from which the desired monosubstituted material can be readily isolated; and, reaction conditions and reagents that are acceptable in large scale production runs. It will be seen from the following descriptions that the methods described herein advance these causes significantly.